1. Field of the Invention
The present invention relates generally to charging circuits used for charging lead-acid batteries and more particularly to a circuit arrangement for bulk charging a lead acid battery in a short time period.
2. Description of Related Art
Rechargeable batteries used in "standby" applications are typically "float charged" from a fixed voltage, regulated, power supply unit (PSU). A 12 volt, regulated, lead acid battery is typically "floated" at 13.65.+-.0.25 volts at 20.degree. C., and when fully charged will draw a few tens of milliamps depending upon capacity, age, etc. While this approach maintains the battery at near 100% capacity, two problems arise when a discharged battery is connected to the PSU, such as when the main AC supply is restored after a prolonged power interruption or when a discharged battery is connected to the PSU.
The first problem is that the PSU has to be designed to provide a significantly higher current than normal to recharge a discharged battery. This requirement increases the size, complexity, cost and dissipation of the PSU just for the infrequent occasions when battery recharge is required. The second problem is that the PSU must be equipped with some means of limiting the battery recharge current to prevent PSU overload, because the lead acid battery has a very low impedance.
One approach to rapidly charging a lead acid battery is described in U.S. Pat. No. 4,609,861 issued Sep. 2, 1986 to Masahira Inaniwa et al wherein a circuit for rapidly charging a lead-acid battery of the sealed type comprises detecting the voltage of the battery, a constant-current charging circuit, a constant-voltage charging circuit, and a timer circuit responsive to the voltage detecting circuit. At the beginning of the charging operation, the battery is charged at a constant current, and when the voltage of the battery exceeds a predetermined value, charging mode is changed to constant-voltage charging which lasts for a predetermined period of time determined by the timer circuit. The charger may also comprise a trickle charge circuit so that trickle charge takes place after the constant-voltage charge. One or more heat-sensitive elements may be used for providing temperature compensation or fail-safe in response to the heat from the battery and heat from parts of the battery charger. A capacitor 50 may be used as a timer which is forcibly charged to insure a given period of time of the constant-voltage charging. A differential amplifier is used for detecting the voltage of the battery so that a power transistor, used as an element of the constant-current charging circuit, is controlled in response to an output signal from the differential amplifier to perform constant-voltage charging. However, this design requires an increased amount of circuitry for accomplishing its objective.
Another U.S. Pat. No. 4,952,861, issued Aug. 28, 1990 to Karl G. Horn describes a three-stage rapid charging process for charging maintenance-free lead batteries with a fixed electrolyte. The process comprises a first charging stage which proceeds at a constant current of the magnitude of a four-hour to eight-hour current until a temperature-dependent charging voltage is reached, a second charging stage which then continues at this temperature-dependent charging voltage for a limited time, and which is continuously adjusted to the battery's temperature, and a third charging stage which is again time-limited and which constitutes a secondary charging which proceeds according to an I/V characteristic curve with an initial charging current limited to between 0.05 and 0.5 times a ten-hour current and a battery voltage which is limited to between 2% and 8% above the temperature-dependent charging voltage. However, this design requires the measurement of temperatures either inside a representative battery cell, between two cells of the battery, or at the terminal connector between two cells.
Another U.S. Pat. No. 5,500,584 issued Mar. 19, 1996 to Kenkuhi Shimomoto describes a battery charging method for charging a lead acid storage battery quickly and a battery charging apparatus. The method includes an initial charging process between times 0 to t.sub.1, where a charging current is increased gradually, a quick charging process between t.sub.1 and t.sub.2 where a charging current is larger than initially, and a final charging process between times t.sub.2 to t.sub.3 where charging current is reduced, carried out step wise and continuous. The battery charging apparatus comprises a primary circuit and a secondary circuit connected by the transformer. Single phase AC is applied to the primary circuit and the secondary circuit outputs connect to the battery. A control switch is turned on and off by a control signal from a control unit. A rectifier performs all wave rectification of the secondary transformer output. A charging current detector detects the terminal voltage of battery. Values from the charging current detector and the terminal voltage detector are fed to the control unit which controls the current supply to the charging circuit controlled by a semiconductor switch. However, this approach requires considerable control electronics.